(1) Field of the Invention
The present invention relates to a gray-scale image display device that performs data writing by sequentially scanning electrodes arranged in matrix.
(2) Related Art
Plasma display panels (PDPs), liquid crystal displays, electroluminescence displays, and the like are the types of matrix displays used in recent years. As an example, a three-electrode surface-discharge AC plasma display device performs data writing in the following manner.
As a method for driving a three-electrode surface-discharge PDP, the so-called address/surface discharge separation method, which separates a write period for writing information in cells to be illuminated and an illumination period for illuminating the cells in which the information has been written, is conventionally known, as disclosed in the Japanese Laid-Open Patent Application No. H07-271325. An operation of the address/discharge sustain separation method in the write period is briefly explained below.
In a PDP 1001 shown in FIG. 10, cells 1002 are the smallest illumination unit. A data electrode group 1003 is arranged vertically, in the order of Xm−1, Xm, and Xm+1 from left to light. A scan electrode group 1004 is arranged horizontally, in the order of Y0, Y1, . . . , Yn−1, Yn, Yn+1 and Yn+2 from top to bottom. A sustain electrode group 1005 is arranged in parallel with the scan electrode group 1004. Each pair of scan and sustain electrodes performs sustain discharge in the illumination period. Note here that m and n are natural numbers.
In this PDP 1001, information is sequentially written starting from the cells positioned on the scan electrode Y0, in the following manner.
First, a scan pulse is applied to the scan electrode Y0, and at the same time a data pulse opposite in polarity to the scan pulse is applied to data electrodes corresponding to cells, among the cells present on the scan electrode Y0, to which information should be written. As a result, the potential difference between the scan and data electrodes in the cells to which information should be written exceeds the discharge firing voltage of the scan and data electrodes, thereby causing a discharge called write discharge to occur in these cells. This write discharge in turn induces a discharge called data sustain discharge between the scan and sustain electrodes in the cells to which information should be written, as a result of which information is written in these cells. Here, the scan and data pulses are never applied simultaneously to the cells to which information is not to be written, so that no write discharge and therefore no data sustain discharge will occur in these cells.
After this, the same operation is performed sequentially on the scan electrodes Y1, Y2, . . . , to the last scan electrode. Thus, scan pulses are applied to all lines.
FIG. 11 shows examples of voltage waveforms which are applied to various electrodes in the PDP 1001 shown in FIG. 10. In the drawing, a data pulse 1100 is applied to the data electrode Xm when a scan pulse 1111 is being applied to the scan electrode Yn, to write information only to the cells where the scan electrode Yn and the data electrode Xm intersect with each other, i.e. cells 1006 diagonally shaded in FIG. 10. Also, a constant voltage is being applied to sustain electrodes throughout the write period.
This being so, when discharge illumination occurring on the data electrode Xm is observed by an oscilloscope using a photodiode or similar by following the scan pulses which are sequentially applied from top to bottom, a waveform 1102 is obtained. This waveform 1102 shows illumination caused by write discharge and data sustain discharge occurring in the cells to which information is to be written. As can be seen from the drawing, discharge occurs only in the cells to which information should be written, while no discharge occurs in the cells to which information should not be written.
In such a matrix display, stray capacitances exist between neighboring data electrodes or between data and scan electrodes. A data electrode drive element charges/discharges these stray capacitances when selecting the cells to which information should be written, and thereby consumes power. A method for reducing the power consumption of the data electrode drive element is disclosed in Japanese Laid-Open Patent Application No. H11-282398. This method changes the order of selecting lines based on the input image data, the power consumption of the data electrode drive element, and the current flowing into the power supply terminal of the data electrode drive element, so as to minimize the power consumption of the data electrode drive element. Suppose horizontal lines are alternately displayed in a panel having 480 lines. If the lines are selected one at a time from top to bottom, the output of the data electrode drive element will end up being inverted 240 times for the 480 lines, so that the stray capacitances of each data electrode will be charged/discharged 240 times. However, if odd-number lines are selected one at a time from top to bottom and then even-number lines are selected one at a time from top to bottom, the output of the data electrode drive element will end up being inverted only once, so that the stray capacitances of each data electrode will be charged/discharged only once. Thus, the power consumption of the data electrode drive element is obviously lower in the latter case than in the former case.
Also, as the line capacity increases due to the recent trend toward high-definition displays, power consumed by charging/discharging electrodes tends to increase. Hence it is necessary to reduce this charge/discharge power consumption, in order to realize lower power consumption for displays.
To reduce the power consumption of the data electrode drive element, conventional techniques decrease the number of times the data electrode drive element is switched by, for example, changing the order of selecting lines for each image. According to this method, however, a simple shift register cannot be used as a scan electrode drive element, which causes complexity in circuit construction.